Flexible and compliant electrodes have been in development for some time. This is due at least in part to the increasing interest in products (e.g., flexible electronic components and “smart” clothing) which require compliant electrodes for providing interconnections between chips and other components.
One area in which compliant electrodes is desirable is in the manufacture of electroactive polymer (EAP) materials for use as “artificial muscles”. EAP materials undergo a strain upon application of a voltage or current, and thus they can be used as actuators. One example of EAP materials is a dielectric elastomer actuator (DEA), which can expand in area up to 300% from a relaxed state when a voltage is applied to compliant electrodes on each face of an elastomer film. Dielectric elastomer actuators are parallel plate capacitors with an elastomeric dielectric between two compliant electrodes. When a large voltage is applied across the electrodes, the two plates are attracted to each other, applying a stress to the elastomeric dielectric between them that is transmitted laterally through Poisson's ratio. These actuators can only function properly when the electrodes are at least as compliant as the elastomeric dielectric.
There are a number of approaches known in the art for making flexible and/or compliant electrodes. One approach is to use carbon grease, which consists essentially of a grease containing carbon particles. The grease material is applied onto both surfaces of an elastomeric material. However, the disadvantage with using grease is that it is not a solid material block or film and, therefore, cannot be used in microfabricated structures or in the construction of shaped materials. In addition, the grease can be easily rubbed from the surface to which it is applied.
Another approach is to produce flexible electrodes consisting of thin layers of metal deposited on the surface of a polymer. Thin film metal electrodes can maintain their conductivity up to tens of percent strain. However, metal films can easily delaminate, particularly at defects, and expensive equipment is typically required to deposit the films.
The strain achieved with thin film metal electrodes can be increased by patterning the films into zig-zag or serpentine designs onto the polymer surface, where the zig-zag pattern is in the plane of the surface. The metal features twist out-of-plane when the polymer is stretched. However, the patterning of metal electrodes on a polymer material, typically performed using photolithography, can be difficult. Polymers can swell in, or react with, solvents and etchants, and the metal may not adhere well to the polymer. In addition, the total area of the device is limited to what can be fit into microfabrication equipment. Still another problem that is prevalent is delamination at the polymer/metal interface during stretching due to the large mismatch of mechanical moduli between the polymer and metal. Another approach to patterning the metal electrodes inplane is to use a shadow mask during metal deposition. While this process reduces complexity, shadow masks can only be used to form relatively thick lines, and the problem of lack of adhesion of the metal to the polymer is still present.
Flexible electrodes formed by metal deposition on a polymer material can also be produced with corrugation of the metal film in the z-direction. For example the polymer material can be stretched prior to depositing the metal film on the surface. Once coated, the stress on the polymer is removed, allowing it to relax to its original shape. This produces a compressive stress on the metal, which therefore wrinkles, creating a corrugated structure on the surface of the polymer. While corrugated surfaces can work well for macro-scale devices, the pre-stretching that is required to form such corrugation would be difficult to implement (and in certain applications impossible) in the formation of micro-scale devices.
Still another approach to forming flexible and/or compliant electrodes is to mix conducting particles (e.g., graphite, carbon nanotubes or silver) into a polymer matrix such as polydimethylsiloxane (PDMS) or polyurethane. A conductive path is made through the material by the particles when the particle concentration reaches the percolation threshold. Advances have been made in producing conductive polymer composites that are compatible with micromachining techniques. For example, graphite and silver particles have been mixed into polyimide and SU-8 matrices to yield conductive polymers that can be incorporated into micromachined devices. In another example, carbon nanotubes have been mixed into PDMS to form deformable capacitor electrodes. In addition, a ternary composite based on polypyrrole, PDMS, and carbon fiber has been tested as a compliant electrode material. However, the major drawback of utilizing this technique is that, as the concentration of particles increases, the elasticity of the material substantially decreases, as determined by a substantial increase in the Young's modulus of the material and/or a reduction in the ultimate strain. In addition, if a photo-patternable polymer is to be employed as the matrix, the mixture loses its ability to be patterned with light if the particles absorb or scatter light at the curing wavelength.
Inherently conductive polymers, or conjugated polymers, have also been mixed into non-conducting host polymers to form compliant electrodes. For example, elastomeric conductors have been formed by mixing polyaniline particles into gel matrices. However, this approach also results in an increase in Young's modulus. Another drawback is that polyaniline absorbs UV light, so this technique cannot be used with most photopatternable polymers.
A further approach for forming an electrically-conductive, stretchable or compliant polymer material is based upon an electrostatic assembly (ESA) technique that is described in U.S. Pat. No. 6,316,084. Using the ESA technique, hundreds of alternating layers of positively charged gold nanoclusters and negatively charged polyelectrolytes are deposited onto a substrate. This substrate can then be removed to yield a free-standing conductive rubber material. While this technique yields a compliant electrode with suitable conductivity and elasticity, it is also time consuming and very expensive.
Ionic polymer metal composites (IPMCs) can also be formed in ion-conducting polymers such as Nafion. For example, in U.S. Pat. No. 4,546,010, a technique is disclosed in which platinum salts are impregnated into an ion-exchange polymer matrix by swelling the polymer and then reducing the salts to achieve a conductive electrode of platinum metal on the ion-exchange surface. While ionic polymer-metal composite electrodes are conductive and flexible, they are not compliant, because the ion conducting material is not elastomeric. In addition, the impregnation step is difficult or impossible to perform in non-ionic polymers such as polydimethylsiloxane (PDMS).
Thus, it is desirable to manufacture a compliant electrode with a suitable conductivity and utilizing a method that is rapid while minimizing cost. It is even more desirable that such a compliant electrode be patternable.